Polyamides are distinguished by a large number of advantageous properties, such as e.g. high toughness, high temperature resistance etc., which guarantee them a secure market position in the engineering thermoplastics sector. These basic properties of the polymer are generally modified by the addition of fillers or additives. Polymer and additives together form the so-called molding composition. Polyamide molding compositions are used in many applications. Injection molded parts, e.g. for the automotive market, or extrudates such as films or hollow articles for the packaging sector may be mentioned as examples.
Films and hollow articles containing a polyamide layer are distinguished by a large number of advantageous properties. Particularly worthy of mention are good optical properties, such as high transparency of films or hollow articles with high surface gloss. Also significant are the good mechanical properties, such as high toughness, high puncture resistance, high tear propagation resistance and others. Ease of production and ease of further processing are added to these.
Of particular significance for many areas of application for films and molding compositions, particularly for use in the packaging sector, e.g. for foodstuffs or cosmetics, is control of the rate of crystallization of the material used, to provide a specific influence on properties such as e.g. shrinkage or impact strength.
In the area of the use of polyamide in the production of films, a fundamental distinction must be drawn between the flat film production process and the blown film production process.
Particularly in the area of application in the blown film sector, polyamides with slower crystallization than conventional polyamide 6 are necessary to enable the primary tube to be blown and stretched in the blown film production process before the film has reached too high a degree of crystallization.
At present, this goal is achieved by the use of copolyamides. The most widespread copolyamide in blown film extrusion is a copolyamide consisting of polyamide 6 and polyamide 66, which usually contains between 15 wt. % and 20 wt. % polyamide 66.
However, other copolyamides with delayed crystallization are also described (e.g. EP-A 561 226). Here, copolyamides of caprolactam, isophthalic acid and hexamethylene diamine with reduced crystallinity compared with conventional polyamides are described. Increased transparency of the multi-layer film claimed is achieved by the reduced crystallinity after processing.
These copolyamides are conventionally produced not by the continuous tubular reactor process, which is widespread for polyamide 6, but by special processes, as described in EP-A 98 412, EP-A 393 546 or WO-A 9421711.
It is known that the transparency of polyamides may be improved by incorporating poly-N-vinylpyrrolidones at the polymerization or compounding stage (DE-A 1 595 613).
The Japanese patent application JP-A 2002306059 describes the coextrusion of a blend of 96% polyamide 6-polyamide 66 copolymer with 4% of a crosslinked N-vinylpyrrolidone to produce packaging for foodstuffs having moderate water vapor permeability and a good oxygen barrier.
The use of poly-N-vinyllactam or poly-N-vinylpyrrolidones is also widespread in the fiber production sector. Here, for example, to increase the hydrophilic properties, 3 wt. % to 15 wt. % of the above compounds are incorporated by compounding. A high proportion of poly-N-vinyllactam or poly-N-vinylpyrrolidone has a negative effect on the yellowness index, however (EP 802 268).
In the area of compounded products, nigrosine base is conventionally used to slow down crystallization, but this leads to a black discoloration of the product.
For applications such as the laser transmission welding of polyamide moldings, materials with the lowest possible crystallinity are needed, since the transmission of laser light decreases with increasing crystallinity. Basic principles of laser transmission welding are described in the specialist literature (Kunststoffe 87 (1997) 3, 348–350; Kunststoffe 88 (1998) 2, 210–212; Kunststoffe 87 (1997) 11, 1632–1640; Plastverarbeiter 50 (1999) 4, 18–19; Plastverarbeiter 46 (1995) 9, 42–46).
A prerequisite for the use of laser beam welding is that the radiation emitted by the laser first passes through a joining partner, which is sufficiently transparent for laser light of the wavelength used, and is then absorbed by the second joining partner in a thin layer of a few 100 μm and converted to heat, which leads to melting in the contact zone and finally to the bonding of the two joining partners by a weld. While it is true that, in the wavelength range of the lasers conventionally used for thermoplastic welding (Nd:YAG laser: 1060 nm; high-performance diode laser: 800–1000 nm), partially crystalline thermoplastics such as polyamides, e.g. polyamide 6 (PA6) and polyamide 66 (PA66) are transparent or laser-translucent, the transmission is often inadequate for good weldability, and so modifications are required for higher transmission.
The nigrosine base does indeed reduce crystallinity, but in the frequency range of 800–1100 nm which is of interest for laser transmission welding, it has marked self-absorptions.
The object of the present invention consequently is to develop a polyamide, preferably polyamide 6, composition in which crystallization is inhibited that is characterized by the absence of any undesirable discoloration.